Lead storage batteries have been closely associated with the development of automotive industry, and against this historical background, efforts have been concentrated on the improvement of starting characteristics, i.e. high rate discharge characteristics, and the cycle life which is related to shallow discharge characteristics. However, as the scope of application of lead storage batteries has remarkably expanded in recent years, with many equipment requiring a high rate discharge have been developed and mounted on vehicles, the compatibility of low rate discharge and high rate discharge characteristics has also become an important consideration.
The aging of characteristics due to deep discharge comprises the early deterioration of capacity that occurs in a comparatively short cycle and the degradation that occurs gradually over a comparatively long time. The early degradation became a prominent problem as lead-calcium-tin alloy and other non-antimony alloys came to be employed with an increasing frequency to meet the market demand for maintenance-free equipment. Under the circumstances the present inventors discovered that the battery structure comprising a less reactive .alpha.-PbO.sub.2 component disposed near the interface between the grid and the active material and a more reactive .beta.-PbO.sub.2 component disposed externally of said less-reactive component was effective in suppressing the above-mentioned early degradation. In addition, as a procedure for accomplishing such a structure, it was proposed that a primary forming stage where pasted electrodes are charged for a short time in an aqueous solution such as an aqueous solution of an alkali such as sodium hydroxide, potassium hydroxide or the like, a neutral aqueous solution of sodium sulfate, potassium sulfate, lithium sulfate or the like, or a dilute aqueous solution of sulfuric acid containing 15 weight % or less of H.sub.2 SO.sub.4, which is suitable for the formation of .alpha.-PbO.sub.2, be conducted prior to the conventional formation in a relatively concentrated aqueous solution of sulfuric acid containing more than 15 weight % of H.sub.2 SO.sub.4.
The above modification of the forming process resulted in a remarkable improvement of the early aging wherein passivation occurs as the discharge is concentrated at the interface between the grid and the active material. However, the aging due to the softening and fall-off of the active material which is manifested after the cycle has progressed fairly far remained yet to be improved. Moreover, this type of aging tended to be localized at the lower portion of the electrode even if, both in the primary and the secondary forming stage, due care was exercised to ensure a uniform treatment of the entire electrode by submerging in a sufficient amount of electrolyte solution. Thus, it was necessary to develop a new method of inhibiting the aging of the lower part of the electrode while maintaining the desirable high rate discharge characteristics.
On the other hand, in the above method which comprises using two dissimilar solutions in the primary and secondary forming stages, a procedure is generally interposed to eliminate the influence of the residual solution from the primary forming stage on the secondary forming stage. Therefore, the prior art teaches the procedures of withdrawing the electrodes from the electrolyte solution after primary formation, discarding the residual solution and washing the electrodes with water. Moreover, for the required formation a charging current of comparatively low current density is applied so as to improve the efficiency of formation without entailing evolution of gases and also to establish a uniform current distribution. In this formation, 0.2 qA is rarely exceeded when the theoretical positive electrode capacity is assumed to be qAh; a value between 0.05 qA and 0.1 qA is common. Notwithstanding the care exercised to ensure a uniform distribution of reaction, the high rate discharge characteristic imparted mainly at the secondary forming stage tends to lack uniformity. Thus, as long as this kind of forming procedure is employed, it is necessary to inhibit a non-uniformity of characteristics, especially the aging of high rate discharge characteristic.
It has thus been desired to further improve the above-mentioned forming method, for ensuring a satisfactory high rate discharge characteristic and inhibiting the early aging, so that the softening and fall-off of the active material which develops concentratedly at the lower part of the electrode in a long run may be alleviated and a uniformity of characteristics in connection with the application of such forming method be assured.